Coherent Rayleigh noise (CRN) acquisition is a technique usable in an optical time domain reflectometry (OTDR) system which allows very small changes in the length and/or temperature of an optical fiber to be detected. As changes in the length of the optical fiber may be related to strain and/or a disturbance imparted on the fiber, a CRN acquisition system can be particularly useful to measure parameters indicative of a problem or potential problem with an elongate structure in proximity to the optical fiber, such as an energy cable, a pipeline, etc. As one example, CRN can be used to detect an encroachment, on an energy cable, for example caused by inadvertent or deliberate digging in the vicinity of the cable. In addition, it could be used to detect partial discharge (PD) through either the vibration induced by PD or the heat generated. Because partial discharge occurs in energy cables prior to complete installation failure, the vibration caused by the partial discharge can be detected via a CRN technique, thus enabling remedial action to be taken prior to damage or failure occurring. CRN also has applications in intrusion detection, as well as in the detection of acoustic events, such as flow-induced noise. Yet further, when used in conjunction with a distributed temperature sensor (DTS) system, data obtained from DTS installations may be refined based on a distributed measurement of vibration or disturbance obtained from the CRN measurement.
To our knowledge, no commercial installations exist where DTS and CRN are used simultaneously. The most common approach to DTS has been Raman OTDR, with some suppliers also offering Raman optical frequency-domain reflectometry (OFDR). Traditionally, DTS measurements over moderate distances (e.g., up to 10 km) have been carried out on multimode fiber, owing to the higher backscatter factor in these fibers, i.e. the higher ratio of usable Raman signal returned to the energy launched into the fiber. There are many thousands of DTS installations worldwide using multimode fiber.
In contrast, it has hitherto been thought that CRN measurements required a single mode fiber to provide a high contrast as a result of a single spatial mode. However, the use of single-mode fiber for DTS measurements degrades the performance (relative to that obtainable on multimode fiber) in Raman systems, at least over moderate distances. Thus an installation requiring DTS and CRN in the same region of interest would generally require two separate fibers: a multimode fiber for DTS measurements and a single-mode fiber for CRN measurements. The installation of separate fibers, however, is not particularly desirable, particularly in terms of cost, as well as in terms of the engineering required to install additional fibers. Alternatively, because the use of multimode fibers for CRN measurements has been avoided since multimode fibers do not produce Rayleigh backscatter that has sufficient contrast for phase changes to be detected by a data acquisition system, a single single-mode fiber could be employed for both DTS and CRN measurements. However, such an arrangement will result in a compromise in the DTS performance. Because of these limitations, pre-existing DTS installations using multimode fiber have been unavailable to use for CRN measurements because of the nature of the fiber in place.